The main antagonist could be a jealous student at Hogwarts. This student sees Harry's connection with the snake as a way to gain power and popularity. So they try to undermine Harry and steal the snake from him, creating a lot of conflict and drama throughout the fanfiction.
It could be a dark wizard who also has an interest in the snake. Maybe he wants to use the snake's powers for evil purposes, like opening a portal to a dark dimension or controlling other magical creatures. So he becomes the main obstacle for Harry as he tries to protect the snake.
Perhaps it's a group of poachers who hunt magical creatures. They know the value of the snake Harry found and are determined to capture it. They will stop at nothing to get their hands on the snake, which makes them the antagonists as Harry battles to keep the snake safe from them.
There could be a group of magical creatures who see the snake as an abomination or a threat to their kind. They would try to destroy the snake and Harry would have to protect it. These creatures could be from a part of the Forbidden Forest that has not been explored much in the original books and they have their own set of rules and magic which makes them a formidable antagonist.
It could be a dark wizard who wants to get his hands on the lightsaber for its power. He might see it as a new tool to use in his evil plans, like using it to break into Gringotts or to fight off any who oppose him.
It could be a dark wizard who also has the ability to communicate with snakes and wants to use this power for evil. He might be jealous of Harry's connection with the snake and try to disrupt it or steal the snake for his own dark purposes.
The main antagonist could be a Slytherin student who has a vendetta against Harry. When they see Harry with the snake, which they believe should belong to a Slytherin, they start causing trouble for Harry. They might try to frame Harry for using dark magic with the snake or try to steal the snake to prove their superiority over Harry.
It could be a dark wizard who also wants to get to the secrets in the lost library. This wizard might try to stop Harry from exploring the library and getting its knowledge.
The antagonist could be a jealous Slytherin student. They might see Harry's new snake as a threat to their own status in Slytherin house. So they try to steal the snake or harm it in some way.
It could be a jealous Slytherin student. Since snakes are often associated with Slytherin, this student might feel that Harry has no right to take care of a snake. They could try to steal the snake or cause trouble for Harry, like spreading rumors that the snake is dangerous and should be removed from Hogwarts.
It could be the person who sent the venomous snake after Harry Potter. Maybe a dark wizard who wants to harm Harry.
In this fanfiction, the main antagonist could be an ancient evil that has been trapped within the grimoire. Once Harry finds the grimoire, this evil starts to gain power and tries to use Harry to break free completely and spread chaos in the wizarding world.
There could be a jealous student. This student might be envious of Harry getting all the attention with his snake - related adventures. So, they would act as an antagonist by trying to undermine Harry's efforts in the snake fanfiction world. They could spread false rumors about snakes being dangerous or Harry's intentions being dark, causing trouble for Harry among his peers and teachers.